High throughput multi beam detection system and method

ABSTRACT

A system and method for inspecting an article, the system includes a spatial filter that is shaped such as to direct output beams towards predefined locations and an optical beam directing entity, for directing the multiple output beams toward multiple detector arrays. The method includes spatially filtering multiple input light beams to provide substantially aberration free output light beams; and directing the multiple output beams by an optical beam directing entity, toward multiple detector arrays.

RELATED APPLICATIONS

This is a DIVISIONAL of, claims priority to and incorporates byreference U.S. patent application Ser. No. 10/910,117, filed 2 Aug. 2004now U.S. Pat. No. 7,326,901, which is a NON-PROVISIONAL of and claimspriority to U.S. Provisional Patent Application No. 60/562,722, filed 15Apr. 2004.

FIELD OF THE INVENTION

The present invention relates to a system and a method for detecting anarray of light spots and especially for allowing high throughputdetection of multiple spots that involves spot spatial distortioncorrection. The array of spots is usually utilized for inspecting anarticle such as but not limited to a wafer, a reticle and the like.

DESCRIPTION OF THE RELATED ART

The inspection of semiconductor wafers is typically performed byscanning a laser beam across a wafer's surface and collecting lightscattered therefrom. The scanning operation is conducted by scanning thelaser beam across the wafer surface in a first direction using one of avariety of known deflectors, such as acousto-optic deflectors orelectromechanical deflectors, while moving a stage that supports thewafer thereon in a second direction, which is typically orthogonal tothe first direction. Another type of inspection includes illuminating anarea and acquiring an image. U.S. Pat. No. 5,699,477 of Alumot et al.and U.S. Pat. No. 6,693,664 of Neumann provide examples of suchinspection systems.

There is a greater emphasis on the throughput of inspection devices andaccordingly on the throughput of scanners, as the design rules forsemiconductors rapidly shrink without a corresponding decrease of theinspection sequence time period or the overall size of semiconductordies or wafers.

High throughput inspection systems utilizes optical beam arrays as wellas electron beam arrays for increasing throughput. Hybrid systems thatinclude electron beam illumination, electro-optical conversion and lightbeam detection are also known. Electron beams also provide higherresolution. U.S. Pat. No. 6,671,042 of Almogy, U.S. Pat. No. 6,639,201of Almogy et al., U.S. Pat. Nos. 6,578,961 and 6,208,411 ofVaez-Iravani, U.S. Pat. No. 6,248,988 of Krantz, which are incorporatedherein by reference; describe state of the art inspection systems.

The beams that form a beam array may be spatially distorted during theillumination as well as during the collection/detection stages of thearticle inspection process. Different beams may be distorted in adifferent manner. Furthermore, the distortion can change over time.

Various image processing methods are known in the art. They includedie-to-die comparison, cell-to-cell comparison and die to databasecomparison. These comparisons require knowledge of the location ofobtained pixels. In other words, spatial distortions can result in acomparison between pixels from different locations of compared dies ofcells.

There is a need for a system and method for compensating for spatialdistortions.

SUMMARY OF THE INVENTION

A system and method for high throughput wafer inspection thatcompensates for various aberrations of an array of light beams.

A system mat includes a spatial filter that is shaped such as to directoutput beams towards predefined locations and an optical beam directingentity, for directing the multiple output beams toward multiple detectorarrays.

A method for inspecting an article, the method includes spatiallyfiltering multiple input light beams to provide substantially aberrationfree output light beams, and directing the multiple output beams by anoptical beam directing entity, toward multiple detector arrays.

A system that includes multiple detector arrays and an optical beamdirecting entity that comprises multiple beam directing elements,whereas the multiple beam directing elements are shaped such as todirect multiple beam portions towards the multiple detector arrays at apredefined manner, substantially regardless of spatial distortions ofthe beams.

A system that includes multiple detector arrays, and an optical beamdirecting entity, for directing multiple beams towards multiple detectorarrays in an interlaced manner.

A method for compensating for spatial aberrations of an array of lightbeams, the method includes: determining a spatial distortion pattern ofthe array of light beams; and configuring a optical component inresponse to the distorted aberrations such as to provide a substantiallynon-distorted array of light beams.

A method for inspecting an article, the method includes: receivingmultiple light beams; and directing, by an optical beam directingentity, multiple beams towards multiple detector arrays in an interlacedmanner.

Other features of the present invention will become apparent from thefollowing detailed description considered in connection with theaccompanying drawings that disclose embodiments of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings. Inthe drawings, similar reference characters denote similar elementsthroughout the different views, in which:

FIG. 1 illustrates an exemplary embodiment of an inspection system,according to an embodiment of the invention.

FIG. 2 illustrates various optical components, according to anembodiment of the invention;

FIG. 3 illustrates a simulated distortion map, according to anembodiment of the invention;

FIG. 4 illustrates a single aperture of the spatial filter and therelationship between the expected beam distortion and the shape and sizeof the aperture; according to an embodiment of the invention;

FIGS. 5 a-5 b illustrates various portions of spatial filters, accordingto various embodiments of the invention;

FIG. 6 illustrates five portions of an intermediate image according toan embodiment of the invention;

FIGS. 7 a-7 b illustrate a portion of a sensor array, according to anembodiment of the invention;

FIG. 8 is a cross sectional view of a Fresnel lens array, according toan embodiment of the invention;

FIG. 9 is a front view of a Fresnel lens array, according to anembodiment of the invention;

FIG. 10 illustrates sixteen spots that are arranged in four rows, aswell as their scan paths, according to an embodiment of the invention,

FIG. 11 illustrates a first line portion that is formed by multiplespots;

FIG. 12 illustrates various optical components, according to anotherembodiment of the invention; and

FIG. 13 illustrates another device for compensating for aberrations,according to an embodiment of the invention.

DETAILED DESCRIPTION

Reference will now be made in greater detail to exemplary embodiments ofthe present invention. In the following description made in conjunctionwith the exemplary embodiments of the present invention, a variety ofspecific elements are described. The following detailed description isof exemplary embodiments of the invention but the invention is notlimited thereto, as modifications and supplemental structures may beadded, as would be apparent to those skilled in the art. Also, in thefollowing description of the present invention, a detailed descriptionof known functions and configurations incorporated herein is omitted.

In particular, but without limitation, while an exemplary embodiment maybe disclosed with regard to the inspection of an article surface bydetecting reflected light using a light source and detecting unit thatare disposed on a common side of an article (a “reflective system”), itwould be readily apparent to one skilled in the art that the teachingsare readily adaptable to the inspection of an article by detectingtransmitted light with a detecting unit that is on a side of an articleopposite to that of the light source (a “transmissive system”). Whilethe reflective system and the transmissive system differ, for oneexample by the absence of a beam splitter in the transmissive system, meprinciples of the present invention are applicable to both types ofsystems. As would be understood by one skilled in the art, both types ofsystems may be utilized separately or together in an inspection of anarticle. Furthermore, it is to be understood that all listed values areprovided for sake of the explanation and are not binding.

FIG. 1 illustrates an exemplary embodiment of an inspection system 100,according to an embodiment of the invention.

The inspection system 100 directs multiple electron beams, such as anarray of 100×100 beams 48 onto an inspected object 52, receivessecondary electrons 49 emitted from the object 52, and eventuallyconverts the array of electron beams to light beams by scintillator 200.System 100 includes an electron source arrangement 20 that generates adiverging beam 22 which is collimated by collimating lens 24 to form asubstantially collimated electron beam 26; This beam illuminates amultiple-aperture arrangement 30 that defined multiple apertures by amultiple aperture plate 28. For simplicity of explanation only fiveapertures were illustrated. The multiple aperture plate 28 can havevarious alternative configurations. It is assumed that it forms astaggered array that includes 10000 apertures. The collimated electronbeam 26 passes through the multiple aperture plate 28 to provide 10000electron beams. The multiple aperture plate further focuses each ofthese electron beams at a first plane, to provide an intermediate imagedenoted by focal points 32. An imaging lens 40 and objective lens 50image the intermediate image onto an article. The imaging lens 40 andobjective lens 50 are oriented in respect to each other, for example byninety degrees, in order to separate the primary beam path from thesecondary beam path. In other words, the beams that pass through imaginglens 40 propagate at a complex path till they pass through objectivelens 50 that in turn focuses them onto the inspected object 52. The pathis “bent” using electromagnetic components, in a manner known in theart. For example, the beams can first pass through an homogenousmagnetic field (represented by first magnetic region 80) deflecting thebeams by a certain angle A to the left. They can than pass a driftregion that is free of magnetic fields (represented by second magneticregion 82). They then can pass through another homogenous magnetic field(represented by third magnetic region 84) that deflects the beams byanother angle B such as to enter the objective lens 50 in about ninetydegrees to the objective lens 50. The secondary beams form a secondaryimage on the surface of the inspected object 52 that is then imaged ontothe scintillator 200. The secondary primary beams propagate across acomplex path, which is “bent” to the right by a third angle C(represented by fourth magnetic region 86).

According to an embodiment of the invention the light beams are arrangedsuch as to form a rectangular array. According to another embodiment thelight beams are arranged such as to form a staggered (or graded) arrayin which even rows and odd rows are misaligned by a predefined distance.For convenience of explanation only the latter formation is illustrated.

FIG. 2 illustrates various optical components, according to anembodiment of the invention. An array of electron beams is directedtowards an electron to light converter, such as scintillator 200.Scintillator 200 provides a distorted array of 100×100 light beams thatare spaced apart from each other by a distance of substantially (due todistortions) of 0.1 mm to form an array of about 10×10 mm. This array ismagnified by an optical arrangement that includes a magnifying lens 210that is followed by an optional field lens 220, a spatial filter 230 anda Fresnel array 235. The optional field lens 220 enables to utilizerelatively concise optical components that are capable of managing beamrows that include many beams.

The 10×10 mm array is magnifies by the magnification lens 210 by afactor often to form an intermediate image 222 of 100×100 mm at fieldlens 220. The magnifying lens 210 has a Numerical Aperture (NA) of 0.5.The space between the magnifying lens 210 and the scintillator 200 isfilled with a fluid that is characterized by an immersion index ofn=1.83.

According to an embodiment of the invention the spatial filter 230 and aFresnel array 235 are positioned in proximity to each other, and theymay also be integrally formed. The spatial filter 230 may be printed onone of the facets of the Fresnel array 235.

The Fresnel array 235 directs various portions of the intermediateimage, via multiple imaging lenses 240-249 towards multiple detectorarrays (also referred to as sensor arrays) 250-259. Each imaging lens240-249 is characterized by a de-magnification factor of 4.

FIG. 3 illustrates a simulated distortion map 300, according to anembodiment of the invention. This map is out of scale. For example, thespacing between undistorted points is about several millimeters whilethe distortion is in the range of microns.

As can be seen by the simulated distortion map 300 instead of a regulararray (illustrated by points 302) a distorted array (illustrated bypoints 310) is formed at the plane of the spatial filter 230. Arrows 320illustrates the distortion. The maximal distortion is denoted MaxD.Lines 304 plots the undistorted grid while lines 303 plots the distortedgrid.

The inventors found that by blocking the perimeter of each input beam,and especially a ring-shaped perimeter that is responsive to MaxD, anoutput beam that passes said blocking will be free of distortions, inthe sense that it will be directed to a predefined location,substantially regardless of the distortion.

Thus, by placing spatial filter 230 at the path of light beams thesystem converts spatial distortions to amplitude variations. In caseswhere the distortions are invariant (time independent) this conversiondoes not introduce an error to die-to-die comparison method and even toother comparison methods.

FIG. 4 illustrates a single aperture 230 _(—) j,k of spatial filter 230.The dashed line 232 represents a cross section of an output beam. Theaperture diameter is smaller than the diameter of the input beam byMaxD. Thus, even if the input beam is distorted by the maximal expecteddistortion, the aperture 230 _(—) j,k passes only a non-distorted outputbeam.

FIGS. 5 a-5 b illustrate various portions of spatial filters 230′ and230″, according to various embodiments of the invention. Spatial filter230′ defines multiple apertures that are arranged in an ordered manner.Spatial filter 230″ includes multiple apertures that are re-arranged inresponse to the shape of the distorted beam array. For example, map 300illustrates that rows that are more distant than an imaginary arraycenter 301 are more oriented in respect to an imaginary horizon thanrows that are more close to the center 301. Rows that belong to theupper half of the array are oriented in a positive angle in relation toan imaginary horizon while rows that belong to the lower half of thearray are oriented in a negative angle in relation to such an imaginaryhorizon.

The inventors found that by positioning the apertures of the spatialarray in oriented rows, responsive to the expected distortions, a largeramount of light can pass through each aperture.

As illustrated by the dashed lines (illustrative of light paths) of FIG.2, each pair of adjacent sensor arrays {(250, 251), (252, 253), (254,255), (256, 257) and (258, 259)} receives beams that are positionedwithin a single portion of the intermediate image 222. As betterillustrated in FIGS. 6-8, this arrangement (also referred to as aninterlaced arrangement) allows utilizing multiple high-speed linesensors. Conveniently, each line of light sensitive elements isconnected to a high-speed shift register.

FIG. 6 illustrates five portions 222_1-222_5 of intermediate image 222.Each pair of sensor arrays receives a corresponding image portion. Afirst portion 222_1 is imaged onto sensor arrays 250 and 251. A secondportion 222_2 is imaged onto sensor arrays 252 and 253. A third portion222_3 is imaged onto sensor arrays 254 and 255. A fourth portion 222_4is imaged onto sensor arrays 256 and 257. A fifth portion 222_4 isimaged onto sensor arrays 258 and 259.

FIGS. 7 a-7 b illustrate a portion 250′ of sensor array 250. Sensorarray 250 includes one hundred line CCDs. Each line CCD is a onedimensional sensor array. Each line CCD includes a row of CCD lightsensitive cells, such as cells 250_1_1-250_1 _(—) j. These cells areconnected in parallel to a high-speed parallel to serial shift register250′_1 that includes one hundred shift register cells, such as shiftregister cells 250′_1_1-250′_1 _(—) j. The shift register cells as wellas the light sensitive cells are positioned on a frontal facet of eachsensor array. The shift register cells form gaps in the light sensitiveareas of each sensor array facet. As illustrated in more details infurther figures, light is directed towards pairs of sensor arrays suchthat a line CCDs of a first sensor array 251 is positioned such as toreceive light that propagates towards shift register cells of anothersensor array. This configuration is referred as an interlacedconfiguration.

FIG. 8 is a cross sectional view of a Fresnel lens array 235, and FIG. 9is a front view of that Fresnel lens array 235, according to anembodiment of the invention. Fresnel lens array 235 includes multiple(such as 100×100) lenses, that form a two dimensional array of lenses.Each lens directs a single beam that passed through an aperture ofspatial filter 230, via an imaging lens, towards a single CCD cell.

According to an embodiment of the invention the spatial filter ispositioned after the Fresnel lens array. For example, a shaped aperturecan be printed to each Fresnel lens.

The lenses of the Fresnel lens array 235 are positioned in accordancewith the paths of substantially non-distorted light beams.

The first twenty rows of Fresnel lenses direct light beams from portion222_1 towards sensor arrays 250 and 251. More specifically, lenses thatbelong to odd rows of the Fresnel lens array 235 direct light beamstowards corresponding line CCDs of sensor array 250 while Fresnel lensesthat belong to even rows of the Fresnel lens array 235 direct lightbeams towards corresponding line CCDs of sensor array 251. It is furthernoted that each row of Fresnel lenses can be translated by a predefineddistance (such as half of a lens length) from an adjacent row of lenses.

The inventors found that by adapting this interlaced arrangement,rectangular CCD arrays can be used.

FIG. 10 illustrates sixteen spots that are arranged in four rows, aswell as their scan paths. A mechanical system moves the imaged articlein a direction which is nearly parallel to an axis (such as an imaginaryy-direction) of the spots such that as the article is moved across thespot array in the scan direction (the y-direction) the spots traces apath which leaves no gaps in the mechanical cross-scan direction (thex-direction). The scan paths are illustrated by dashed lines. The firstrow includes spots 400_1_1-400_1_4. These spots scan an article alongscan paths 401_1_1-401_1_4. The second row includes spots400_2_1-400_2_4. These spots scan an article along scan paths401_2_1-401_2_4. The third row includes spots 400_3_1-400_3_4. Thesespots scan an article along scan paths 401_3_1-401_3_4. The fourth rowincludes spots 400_4_1-400_4_4. These spots scan an article along scanpaths 401_4_1-401_4_4.

The most upper row is the fourth row, and it is followed by ninety-sixrows to provide an array that includes one hundred rows. Each rowfurther includes ninety-six spots to provide a row of one hundred spots.

Within the system various images of these spots are provided. A firstimage is formed on the inspected article. Another image is formed at thescintillator plane. Yet another image (previously referred to as anintermediate image) is formed at the spatial filter. This intermediateimage 222 is split and de-magnified to provide multiple image portionson the sensor arrays. For simplicity of explanation the followingdescription will refer to the intermediate image.

The intermediate image is about 100×100 mm. The distance betweenadjacent spots that belong to the same row is 1 mm. The verticaldisplacement between adjacent rows is 1 mm. The spots of the odd rowsare horizontally displaced in relation to corresponding spots of theeven rows by 0.5 mm.

An imaginary horizontal line 403 is crossed by the scan lines401_1_1-401_4_4 at multiple imaginary points denoted P1,1-P4,4. Theimaginary horizontal line 403 is first crossed by spots 400_1_1-400_1_4,than by spots 400_2_1-400_2_4, followed by spots 400_3_1-400_3_4 andfinally is crossed by spots 400_4_1-400_4_4.

Due to the horizontal displacement between rows the spot array includestwo hundred columns, each column includes fifty spots. The odd columnsinclude spots that define the odd rows and the even columns includespots that define the even columns. For example, spots 400_1_1 and400_3_1 define a first column and belong to odd rows. Each columncrosses the imaginary horizontal line 403 to form a continuous lineportion.

Due to the vertical displacement between rows, the even and odd rowscross the imaginary horizontal line 403 in an interlaced manner. Byusing interlaced sensor arrays, each sensor array receives a stream ofspots that form a continuous line portion.

FIG. 11 illustrates a first line portion 403_1 that is formed by pointsP1,1; P3,1; P5,1; P7,1 . . . P47,1 and P49,1. A first line portion 403_1is followed by second line portion 403_2 that is formed by points P2,1;P4,1; P6,1; P8,1 . . . P48,1 and P50,1. The first line portion 403_1 isimaged onto a single cell of a line CCD of sensor array 250 while thesecond line portion 403_2 is imaged onto a single cell of a line CCD ofsensor array 251. Each cell of the CCD line array receives a continuousstream of light beams that form a continuous line.

The duty cycle of the line CCDs can reach almost 100%. State of the artline CCDs are characterized by an integration period T1 that is muchlonger than the discharge time from CCD cell to shift register cell T2;During an integration period the content of the shift register is sentto a storage unit.

According to another embodiment of the invention the spot array isrectangular shaped and includes one hundred rows and one hundredcolumns. Thus, the first imaginary line portion 403_1 will be formed byfifty spots of the first column P1,1-P1,50 and the second imaginary lineportion 403_2 is formed by other fifty spots of the same first columnP1,51-P1,100.

FIG. 12 illustrates various optical components, according to anotherembodiment of the invention. The optical components include a beamsplitter and large objective lenses instead of the Fresnel lens array.

A Scintillator 200 is followed by a magnifying lens 210 that in turn isfollowed by a beam splitter 510, optional field lenses 520 and 539,spatial filters 522 and 532, multiple imaging lenses 240-249 andmultiple sensor arrays 550-559. The various optical componentsfacilitate imaging of the whole light beam array, and compensate for thedifferences between the size of sensor array facets and the size of thelight sensitive cells. Assuming that an intermediate image 560 is formedat the beam splitter 510 then the odd portions of that image 560_1,560_3, . . . 560_9 are imaged by sensor arrays 551, 553, 555, 557 and559 that are positioned to receive light that passes through field lens520. The even portions 560_0, 560_2, . . . 560_8 are imaged by sensorarrays 550, 552, 554, 556 and 558 that are positioned to receive lightthat passes through field lens 539.

The sensor arrays may include CCD arrays, such as backside illuminatedCCD arrays, but this is not necessarily so.

The beam splitter 510 directs an intermediate image of light beamstowards each of the objective lenses 520 and 539. Spatial filters 522and 532 may be positioned in proximity to the field lenses but they mayalso be replaced by a single spatial filter that is positioned along apath of the light beams, between the magnifying lens 210 and the beamsplitter 510.

FIG. 13 illustrates another device for compensating for aberrations. Thedevice 700 includes two arrays 710 and 720 of microlenses that face eachother. The first microlens array 710 is shaped such that the location offocal points of the lenses corresponds to a distorted array, while thesecond array 720 is shaped such that the focal points of the lens forman ordered array.

The images can be processed by applying various comparison methods, suchas die to die comparison, cell-to-cell comparison and die to databasecomparison.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment. Rather, it is intended to cover variousmodifications within the spirit and scope of the appended claims.

1. A method, comprising: configuring an optical component in response to a determination of a spatial distortion of an array of light beams so as to provide a substantially non-distorted array of light beams, wherein the optical component comprises two microlens arrays and configuring the optical component comprises configuring microlenses of a first one of the microlens arrays so that a location of focal points of the microlenses of the first microlens array corresponds to a distorted array, and configuring microlenses of a second of the microlens arrays so that a location of focal points of the microlenses of the second microlens array corresponds to a substantially non-distorted array. 